This invention relates to the manufacture of metallic tubing. It is more particularly concerned with a method for manufacturing thin-walled zinc coated electrical metallic tubing, fence post tubing, ladder railings and the like, and with the tubing so produced.
The uses of fence post tubing, ladder railings and the like are self explanitory and are well known. Generally such tubing is produced by the same process as described for electrical metallic tubing. Electrical metallic tubing is used for metal raceways for the installation of electrical wires and cables. Large quantities of this tubing are made of steel in size ranges from nominal 3/8 inch to nominal 4 inch diameter. The tubing is relatively thin-walled, having a wall thickness of about 0.042 inch in the smallest sizes and increasing to about 0.083 inch for nominal 4 inch tubing. Steel tubing of this type is conventionally made by forming a flat blank into a tube and welding the edges together. The smaller sizes of electrical metallic tubing are often bent in fabrication and must be able to withstand bending without cracking, rupture or collapse. Tubing of 1/2 inch nominal or trade size, for example, must be able to withstand bending into a semi-circle the inner edge of which has a radius of 31/2 inches and subjected to a hydrostatic pressure of 30-50 psi to test for seam cracks or openings.
Steel tubing of this type is commonly protected by a zinc coating. The tubing after forming and welding is conventionally hot dip galvanized, electro-galvanized or seam metallized so that the weld is coated to the same extent as the remainder of the surface. The Underwriters' Laboratories require that the galvanized coating on the exterior of the tubing meet certain thickness standards described hereinafter. These standards do not apply to the coating on the inside of the tube, but that coating must protect the tubing against corrosion. In practice, therefore, the inside surface of the formed tubing is spray or flow coated with an enamel, paint or other acceptable coating.
The coating thickness test specified by the Underwriters' Laboratories is commonly known as the Preece test and is described in detail in the Underwriters' Laboratories standards for electrical metallic tubing. UL 797. Specimens of the zinc coated steel are immersed or dipped in a copper sulfate solution of prescribed strength for sixty seconds and are then removed and washed in running water. The zinc from the specimen displaces copper from the solution, which plates out on the specimen. The copper does not adhere strongly to zinc, however, and the loosely adhering deposits are removed by washing in water, followed by wiping the specimen with cheesecloth. The procedure described is then repeated, to an end point described hereinafter. The coating thickness is determined by the number of successive dips which the coating can withstand without dissolving to the steel base. When the zinc is removed down to the iron, which also displaces copper from the solution, the copper adheres firmly to the iron and cannot be washed or rubbed off. The zinc coating of electrical metallic tubing must withstand four such immersions or dips without showing a final firm deposit of copper.